JP2016140783A - Organic wastewater treatment method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic wastewater treatment method and apparatus which can avoid fouling of a membrane separator used in a membrane separation step even if the circulation ratio of a nitrified liquid to a denitrification step is set to a small value.SOLUTION: An organic wastewater treatment method that performs biological treatment of organic wastewater containing nitrogen as water to be treated having been mixed with activated sludge by at least a denitrification step, a nitrification step, and a membrane separation step using an immersion type membrane separator in this order includes: a first return step of returning at least the water to be treated not having been taken out as a membrane permeate to remain in the membrane separation step; a second return step of returning the water to be treated, not having been taken out as the membrane permeate to remain in the membrane separation step, to a step subsequent to the denitrification step; and a return flow rate adjustment step of adjusting the flow rates of the water to be treated returned in the first and second return steps using as an index the nitrogen concentration of one of the organic wastewater, either water to be treated, and the membrane permeate membrane-separated in the membrane separation step.SELECTED DRAWING: Figure 1

Description

  The present invention organically treats organic wastewater containing nitrogen as treated water mixed with activated sludge in the order of at least a denitrification step, a nitrification step, and a membrane separation step using a submerged membrane separation device. The present invention relates to a wastewater treatment method and a treatment apparatus therefor.

  Patent Document 1 includes a biological treatment tank in which a denitrification part having a stock solution inflow part and a nitrification part having a treatment liquid outflow part are in communication with each other, and an oxygen-containing gas is introduced into or outside the treatment tank. An air lift unit with a pipe is installed, and a recycle route is provided with a circulation channel for forcibly transferring the denitrification liquid in the denitrification unit to the nitrification unit and a communication channel for allowing the nitrification solution in the nitrification unit to flow into the denitrification unit And a biological denitrification apparatus that improves the denitrification rate is disclosed. A nitrification step in which ammonia is nitrified in the nitrification unit is executed, and a denitrification step in which nitrate nitrogen is reduced and separated as nitrogen gas is executed in the denitrification unit in which the nitrification liquid is returned.

  According to Patent Document 2, along with the aging of the sewage treatment apparatus adopting the conventional standard activated sludge method constructed for the purification treatment of sewage such as general municipal sewage and industrial wastewater such as domestic wastewater, It is disclosed that refurbishment to a sewage treatment apparatus using a membrane separation activated sludge method is in progress in order to effectively remove phosphorus, nitrogen and the like from sewage. By providing a membrane separation tank instead of the precipitation tank, the A-SRT can be lengthened and the nitrification treatment can be promoted.

  In Patent Document 3, as shown in FIG. 5, an anaerobic tank, a denitrification tank, and an aeration tank in which a separation membrane is immersed are placed in the order of biological treatment equipment, and waste water is introduced into an anaerobic tank upstream. A wastewater treatment method using a membrane separation activated sludge method for treatment is disclosed.

  In the wastewater treatment method, wastewater is passed in the order of a phosphorus release step, a denitrification step, and an aeration step in the presence of activated sludge, and the water to be treated is separated by a separation membrane immersed in the liquid mixture of the aeration step. In the wastewater treatment method, a part of the liquid mixture in the aeration process is returned to the denitrification process, and the liquid mixture in the denitrification process is returned to the phosphorus release process.

  A part of the mixed solution nitrified to nitrate nitrogen and nitrite nitrogen in the aeration process is returned to the denitrification process, reduced to nitrogen gas, separated and removed, and deoxygenated in the denitrification process Is returned to the phosphorus release step and the phosphorus compound is released as normal phosphoric acid. The released orthophosphoric acid is taken up by aerobic microorganisms in the aeration process.

  When using the membrane separation activated sludge method described above, the organic wastewater containing nitrogen is treated with activated sludge in the order of at least the denitrification step, the nitrification step, and the membrane separation step using the submerged membrane separation device. It is biologically treated as water, and if necessary, a phosphorus release step is incorporated before the denitrification step.

  In order to reduce the concentration of nitrogen contained in the treated water after biological treatment, the nitrification liquid circulation ratio R to the denitrification process of the water treated in the nitrification process should be increased. R is set between 2 and 4.

The nitrification liquid circulation ratio R is nitrification when the flow rate of treated water is 1 or the flow rate of treated water (membrane permeated water drained from the membrane separation device when using the membrane separation activated sludge method) is 1. The treated water nitrogen concentration, which is the ratio of the circulation rate of the liquid and is the nitrogen concentration of the membrane permeated water, is obtained by the following formula 1. The nitrogen concentration to be nitrified is a value obtained by removing nitrogen components extracted as surplus activated sludge from the total nitrogen contained in the organic wastewater as raw water.
(Treatment water nitrogen concentration) = (nitrogen concentration to be nitrified) / (R + 1) (1)

JP 55-45357 A JP 2013-664 A JP 2001-314890 A

  The treated water that has been biologically treated by the sewage treatment equipment using the membrane separation activated sludge method is subjected to advanced treatment such as COD removal as necessary, and then discharged into rivers and flow down into the sea. From this point of view, it may not always be preferable to adjust the treated water nitrogen concentration to a low value.

  For example, the concentration of nutrients in seawater, especially dissolved inorganic nitrogen, is said to have an effect on cultured seaweed, and the dissolved inorganic nitrogen necessary for pigment synthesis of cultured seaweed from winter to spring, when the seaweed is growing. Due to the shortage, there is a problem that the laver discoloration phenomenon occurs.

  In order to adjust the nitrogen concentration of the treated water to a somewhat high value from winter to spring, an organic wastewater treatment method in which the nitrification liquid circulation ratio R in Formula 1 is reduced has been tried.

  However, when the nitrification liquid circulation ratio R is set to a small value, the amount of water to be treated entering the membrane separation process decreases, and the activated sludge concentration around the membrane separation apparatus disposed along the flow of the water to be treated As a result, there was a problem that the variation in the density of the liquid crystal became large and the membrane separation performance was hindered.

When the concentration of the activated sludge suspended matter MLSS to be treated returned from the membrane separation process to the denitrification process is X mg / L, the MLSS concentration flowing into the membrane separation process is obtained by the following formula 2. Based on Formula 2, it is understood that the smaller the nitrification liquid circulation ratio R, the larger the difference in MLSS concentration between the treated water flowing into the membrane separation process and the treated water returned from the membrane separation process to the denitrification process. it can.
MLSS (mg / L) = R / (R + 1) * X Equation 2

  In other words, the MLSS concentration is increased on the downstream side along the flow of the water to be treated in the membrane separation step, and activated sludge is deposited on the separation membrane of the membrane separation device, which may cause membrane separation.

  In addition, if the MLSS concentration in the entire treatment process is lowered so that the MLSS concentration on the downstream side along the flow of the water to be treated in the membrane separation process does not become too high, especially on the upstream side of the membrane separation process in which the MLSS concentration is low. There is a possibility that undecomposed ammonia or the like flows out into the permeated water of the membrane, or fouling of the separation membrane due to undegraded hardly soluble components or polymer solutes occurs and the separation membrane is clogged.

  In view of the above-mentioned problems, the object of the present invention is an organic wastewater capable of avoiding fouling of a membrane separation apparatus used in a membrane separation process even if the nitrification liquid circulation ratio to the denitrification process is set to a small value. It is in the point which provides the processing method and its processing apparatus.

  In order to achieve the above-mentioned object, the first characteristic configuration of the organic wastewater treatment method according to the present invention is to at least remove organic wastewater containing nitrogen as described in claim 1 of the claims. A method for treating organic wastewater that is biologically treated as treated water mixed with activated sludge in the order of a nitriding step, a nitrification step, and a membrane separation step using a submerged membrane separation device. A first return step for returning the water to be treated that has not been taken out as water to the denitrification step, and a step after the denitrification step for the water to be treated that has not been taken out as membrane permeated water in the membrane separation step Returned in the first return step, using as an indicator the nitrogen concentration of the second return step to return to the organic waste water, any of the water to be treated, and the membrane permeated water separated in the membrane separation step Flow rate of water to be treated, and the second In that it includes a return flow rate adjusting step for adjusting the flow rate of the water to be treated, the sending back by the feeding step.

  According to the above-described configuration, the nitrogen removal rate can be adjusted based on the return amount of the water to be treated which is not taken out as membrane permeated water in the membrane separation step returned in the first return step, but the second removal rate can be adjusted. The concentration of activated sludge flowing into the process after the denitrification process can be adjusted by the return amount of the water to be treated that is not taken out as the membrane permeate in the membrane separation process returned in the return process. In other words, in the return flow rate adjustment step, the return of the treated water returned in the first return step using the organic drainage as raw water, the treated water during biological treatment, or the nitrogen concentration of membrane permeated water as an index. The amount is adjusted, and the return amount of the water to be treated returned in the second return step is adjusted to an appropriate value according to the return amount.

  In the second characteristic configuration, as described in claim 2, in addition to the first characteristic configuration described above, the return flow rate adjustment step uses the nitrogen concentration as an index to determine the nitrogen concentration of the membrane permeated water. In the second return step, the flow rate of the water to be treated returned in the first return step is adjusted so as to become the target nitrogen concentration, and the flow rate of the water to be treated flowing into the membrane separation step becomes the target flow rate. This is a process for adjusting the flow rate of the water to be returned.

  In the return flow rate adjustment process, the nitrification liquid necessary to obtain the target treated water nitrogen concentration using the nitrogen concentration of organic wastewater as raw water, treated water during biological treatment, or membrane permeated water as an index The circulation ratio R is obtained, and the return amount of the water to be treated returned in the first return step is adjusted based on the nitrification solution circulation ratio R. Furthermore, the return amount of the treated water returned in the second return step is adjusted to an appropriate value according to the return amount. Therefore, it is possible to avoid occurrence of fouling of the separation membrane in the membrane separation step regardless of the value of the treated water nitrogen concentration.

  In the third feature configuration, in addition to the second feature configuration described above, the target flow rate is more than three times the inflow amount of the organic waste water to be treated and 6 It is in the point which is adjusted to the range below double.

  When the flow rate of the water to be treated flowing into the membrane separation step is adjusted to a range of 3 to 6 times the inflow of organic waste water as raw water, the MLSS concentration distribution ratio in the membrane separation tank is set to 1 : It becomes possible to be within the range of 1.2 to 1: 1.5. As a result, generation of fouling of the separation membrane can be effectively suppressed even when the flow rate of water to be treated returned in the first return step is reduced and the nitrogen concentration of the membrane permeated water is set high. A separation membrane that can handle the MLSS concentration distribution ratio in the range of 1: 1.2 to 1: 1.5 is standard and easily available at low cost. Furthermore, since the control range of the flow rate of the water to be treated flowing into the membrane separation process is relatively wide, ranging from 3 to 6 times the flow rate of the organic waste water to be treated, it is very difficult to control the flow rate. Without.

  In the fourth feature configuration, in addition to the second feature configuration described above, the target flow rate is three times or more and six times the inflow amount of the organic waste water to be treated. The point is that it is adjusted to a constant value in the following range.

  When the flow rate of treated water flowing into the membrane separation process is adjusted to a constant value in the range of 3 to 6 times the inflow of organic wastewater that is the raw water, the MLSS concentration distribution ratio in the membrane separation tank Can be kept within a preferable constant value in the range of 1: 1.2 to 1: 1.5. As a result, even when the flow rate of the treated water returned in the first return process is reduced and the nitrogen concentration of the membrane permeated water is set higher, the occurrence of fouling of the separation membrane can be more effectively suppressed. .

  The first characteristic configuration of the organic wastewater treatment apparatus according to the present invention is, as described in claim 5, at least a denitrification tank that biologically treats organic wastewater containing nitrogen as treated water mixed with activated sludge, An organic wastewater treatment apparatus comprising a nitrification tank and a membrane separation tank provided with a submerged membrane separation apparatus, wherein the first return mechanism returns the treated water in the membrane separation tank to the denitrification tank And a second return mechanism for returning the treated water in the membrane separation tank to the subsequent stage from the denitrification tank, the organic waste water, any treated water, and the membrane permeation taken out by the submerged membrane separation apparatus. A nitrogen concentration measuring device for measuring any nitrogen concentration of water, a flow rate of water to be returned to the first returning mechanism using the nitrogen concentration acquired by the nitrogen concentration measuring device as an index, and the first Processed to be returned via the two-return mechanism In that it includes a control unit for adjusting the flow rate.

  In the second feature configuration, as described in claim 6, in addition to the first feature configuration described above, the control unit uses the nitrogen concentration acquired by the nitrogen concentration measurement device as an index. The flow rate of the water to be treated returned through the first return mechanism is adjusted so that the nitrogen concentration of the permeated water becomes the target nitrogen concentration, and the flow rate of the water to be treated flowing into the membrane separation tank becomes the target flow rate. In this way, the flow rate of the water to be treated returned through the second return mechanism is adjusted.

  In the third feature configuration, in addition to the second feature configuration described above, the target flow rate is more than three times the inflow amount of the organic waste water to be treated and 6 It is in the point which is adjusted to the range below double.

  In the fourth feature configuration, in addition to the second feature configuration described above, the target flow rate is at least three times and six times the inflow amount of the organic waste water to be treated. The point is that it is adjusted to a constant value in the following range.

  The fifth characteristic configuration further includes an MLSS concentration measuring device that measures the MLSS concentration of the water to be treated in the membrane separation tank, in addition to the first characteristic configuration described above, as described in claim 9. The control unit uses the nitrogen concentration acquired by the nitrogen concentration measuring device as an index, and the treated water is returned via the first return mechanism so that the nitrogen concentration of the membrane permeated water becomes the target nitrogen concentration. While adjusting the flow rate, the second return according to the flow rate of the water to be treated returned through the first return mechanism so that the MLSS concentration acquired by the MLSS concentration measuring device falls within a predetermined range. It exists in the point which adjusts the flow volume of the to-be-processed water returned via a mechanism.

  Since the MLSS concentration of the treated water in the membrane separation tank is directly monitored and the flow rate of the treated water returned through the second returning mechanism is adjusted so that the value falls within a predetermined range, the first return Even when the flow rate of the water to be treated returned in the process is reduced and the nitrogen concentration of the membrane permeated water is set to be high, generation of fouling of the separation membrane can be more effectively suppressed. Moreover, since the possibility of fouling of the separation membrane can be indirectly detected and prevented based on the acquired MLSS concentration, it is possible to lengthen the time until regular maintenance due to fouling of the separation membrane.

  In addition to any one of the first to fifth feature configurations described above, the sixth feature configuration is the above-described first to fifth feature configuration. A plurality of submerged membrane separation devices are arranged, and the first return mechanism is provided with a return path for returning treated water downstream of the membrane separation tank to the denitrification tank.

  According to the above-described configuration, even if the flow path of the water to be treated flowing in the membrane separation tank has a long shape such that a plurality of submerged membrane separation devices are arranged along the flow direction of the water to be treated. Since the MLSS concentration gradient along the flow direction of the treated water can be reduced, a substantially uniform membrane separation process can be performed from the upstream side to the downstream side of the membrane separation tank.

  As described above, according to the present invention, even if the nitrification liquid circulation ratio to the denitrification step is set to a small value, the organic wastewater treatment that can avoid fouling of the membrane separation device used in the membrane separation step. It has become possible to provide a method and a processing apparatus thereof.

Explanatory drawing of the organic waste water treatment apparatus and treatment method by this invention Explanatory drawing of the organic waste water treatment apparatus and treatment method which show another embodiment Explanatory drawing of the organic waste water treatment apparatus and treatment method which show another embodiment Explanatory drawing of the organic waste water treatment apparatus and treatment method which show another embodiment Explanatory drawing of conventional waste water treatment equipment and treatment method

  Hereinafter, embodiments of an organic wastewater treatment method and treatment apparatus according to the present invention will be described.

  As shown in FIG. 1, the organic wastewater treatment apparatus 1 is an apparatus that sequentially biologically treats organic wastewater containing nitrogen as raw water as treated water mixed with activated sludge, and at least a denitrification tank 20. , A nitrification tank 21, and a membrane separation tank 22 provided with a submerged membrane separator 23.

  Nitrogen-containing organic wastewater is typically wastewater containing municipal and sewage, human waste from which solids have been removed, and organic and ammonia components discharged from food factories and the like. The stage before the denitrification tank 20 is usually provided with a screen mechanism for removing impurities contained in the organic wastewater, a pretreatment tank for removing solids contained in the organic wastewater, etc., but this embodiment will be described. Is omitted.

  The denitrification tank 20 is provided with a stirring mechanism 20a, and is configured so that organic wastewater as raw water is mixed with activated sludge under anaerobic conditions. In the denitrification tank 20, the water to be treated is anaerobically treated by a facultative anaerobic microorganism (here, denitrifying bacteria) contained in the activated sludge, and nitrate nitrogen and nitrite nitrogen contained in the water to be treated are reduced to nitrogen gas. In other words, it is denitrified. Reduced nitrogen is released into the atmosphere.

  The nitrate nitrogen and nitrite nitrogen are treated by aerobic treatment of the ammonia component contained in the raw water by the aerobic microorganisms contained in the activated sludge in the nitrification tank 21 and the membrane separation tank 22, and oxidized into nitric acid and nitrous acid. The treated water is contained in the treated water returned via the first return mechanism A described later.

  The nitrification tank 21 is provided with an aeration device 25 and configured to aerate the water to be treated. Ammonia contained in the treated water that has been aerated is oxidized to nitrite nitrogen by nitrite bacteria contained in activated sludge under aerobic conditions, and nitrite is oxidized to nitrate nitrogen by nitrate bacteria. It is. Moreover, the organic component contained in to-be-treated water is decomposed by aerobic microorganisms.

  A plurality of submerged membrane separators 23 are immersed in the membrane separation tank 22 along the flow direction of the water to be treated, and solid matter such as sludge is separated from the water to be treated by the submerged membrane separator 23. It is comprised so that membrane permeated water may be obtained. The treated water is pumped from the upstream treatment tank to the downstream treatment tank, or the treated water overflows from the upstream side to the downstream side of the adjacent treatment tank.

  The submerged membrane separation device 23 incorporates a plurality of membrane units 24 adopting a separation membrane made of a flat membrane-shaped microfiltration membrane, and a header pipe 31 a is connected to a drain pipe of each membrane unit 24. Membrane permeated water is sucked by a suction pump 31 connected to the header pipe 31a, and after advanced treatment is performed as necessary, it is discharged into a river.

  Examples of separation membranes that can be used in the submerged membrane separation device 23 include ultrafiltration membranes and nanofiltration membranes in addition to microfiltration membranes, and examples of separation membranes include hollow fiber membranes, tubular membranes, etc. in addition to flat membranes. Can be mentioned. In FIG. 1, the air diffuser 25 is shown as being disposed at the bottom of the membrane separation tank 22, but in reality, the air diffuser is formed below the membrane unit 24 accommodated in each submerged membrane separator 23. An apparatus 25 is provided, and the upward flow of water to be treated formed by bubbles diffused from each of the air diffusers 25 prevents the sludge from adhering to the surface of the membrane unit 24 and removes the adhering sludge. Further, the water to be treated is aerobically treated.

The organic waste water treatment apparatus 1 is further provided with a first return mechanism A, a second return mechanism B, a nitrogen concentration measuring device 33 and a control unit 30.
The first return mechanism A includes a first return path 28 for returning the water to be treated that has not been subjected to membrane separation treatment downstream of the membrane separation tank 22 to the upstream side of the denitrification tank 20, and a first return path 28. A circulation pump (hereinafter referred to as “nitrification liquid circulation pump”) 26 for feeding the treated water in the membrane separation tank 22 and a flow meter 32a for measuring the flow rate of the treated water flowing through the first return path 28 are provided. It is prepared for. As the nitrification liquid circulation pump 26, any of a submersible pump provided with an impeller driven by an electric motor or the like, an air pump that pumps water by an upward flow of air supplied from a blower, and the like may be adopted.

  The second return mechanism B is separated into the second return path 29 for returning the water to be treated which has not been subjected to the membrane separation process on the downstream side of the membrane separation tank 22, and the second return path 29. A circulation pump (hereinafter referred to as “internal circulation pump”) 27 for feeding the water to be treated in the tank 22 and a flow meter 32b for measuring the flow rate of the water to be treated flowing through the second return path 29 are configured. ing. The internal circulation pump 27 may employ any of a submersible pump, a land pump, and an air lift pump, like the nitrification liquid circulation pump 26.

  In this embodiment, the nitrogen concentration measuring device 33 is configured to measure the nitrogen concentration contained in the raw water. As will be described later, the nitrogen concentration measuring device may be provided in the denitrification tank 20 (see reference numeral 33a) or in the nitrification tank 21 (see reference numeral 33b), as indicated by a broken line in FIG. And the submerged membrane separation device 23 (see reference numeral 33c), and further, the nitrogen concentration of the membrane permeated water may be measured (see reference numeral 33d).

  The control unit 30 is configured by, for example, a personal computer device, a microcomputer, an FPGA, or the like, and an input unit that inputs nitrogen concentration data acquired by the nitrogen concentration measuring device 33 and flow rates acquired by the flow meters 32a and 32b. , The output section for driving the nitrification liquid circulation pump 26 and the internal circulation pump 27, and the circulation of the water to be treated through the nitrification liquid circulation pump 26 and the internal circulation pump 27 based on the nitrogen concentration and each flow rate input to the input section. And an arithmetic unit for calculating the quantity.

  The calculation unit calculates a flow rate of the water to be treated returned by the first return mechanism A and a flow rate of the water to be treated returned by the second return mechanism B using the nitrogen concentration input via the input unit as an index. The nitrification liquid circulation pump 26 and the internal circulation pump 27 are adjusted so that the flow rate of each treated water input via the flow rate calculation unit and the input unit becomes the flow rate of the treated water calculated by the target flow rate calculation unit. A return flow rate adjustment unit is provided.

  That is, the control unit 30 uses the nitrogen concentration acquired by the nitrogen concentration measuring device 33 as an index, the flow rate of the water to be treated returned through the first return mechanism A, and the target to be returned via the second return mechanism B. It is comprised so that the flow volume of a treated water may be adjusted. As a result, the nitrogen concentration of the membrane permeated water and the MLSS concentration which is the concentration of activated sludge in the membrane separation tank 22 are adjusted to preferable values.

Hereinafter, the nitrogen concentration of the permeated water adjusted by the control unit 30 and the MLSS concentration of the membrane separation tank 22 will be described in detail.
Part of the water to be treated in the membrane separation tank 22 is returned to the denitrification tank 20 via the first return mechanism A. In this specification, this flow of water to be treated is called nitrification liquid circulation. Further, part of the water to be treated in the membrane separation tank 22 is returned to the nitrification tank 21 via the second return mechanism B. In the present specification, this flow of water to be treated is called internal circulation.

  The ratio of the flow rate of the treated water circulating in the nitrification solution to the inflow amount 1 of the raw water flowing into the denitrification tank 20 is the nitrification solution circulation ratio R, and the ratio of the flow rate of the treated water circulating internally to the flow rate of the raw water 1 is also internally circulated. Let the ratio be r.

  Nitrogen contained in the raw water exists mainly as ammonia nitrogen and is nitrified in the nitrification tank 21 to change into nitrate nitrogen or the like. The amount of nitrogen contained in the water to be treated is not greatly changed by nitrification. The amount of nitrogen in the water to be treated changes in the course of the raw water being treated with activated sludge when nitrate nitrogen is reduced in the denitrification tank 20 and released into the atmosphere as nitrogen gas. This is when nitrite nitrogen, nitrate nitrogen, and the like are extracted as excess sludge in the tank 22.

Of the nitrogen content that is removed, the nitrogen content that is extracted as excess sludge is small, so the nitrogen concentration that should be removed by nitrification / denitrification treatment in the concentration of nitrogen contained in the raw water is the nitrification target nitrogen concentration and the nitrogen content of the membrane permeated water. When the concentration is the treated water nitrogen concentration, the treated water nitrogen concentration can be calculated by the following Equation 1 as described above.
(Treatment water nitrogen concentration) = (nitrogen concentration to be nitrified) / (R + 1) (1)

  If any one of the treated water nitrogen concentration, nitrification target nitrogen concentration, and nitrification liquid circulation ratio R is known, the remaining values are also found. The measured value of the nitrogen concentration of the membrane permeated water becomes the treated water nitrogen concentration, the measured value of the raw water nitrogen concentration becomes the nitrification target nitrogen concentration, and the flow rate of the membrane permeated water and the return amount of the treated water via the first return mechanism A The ratio becomes the nitrification liquid circulation ratio R.

  The nitrification liquid circulation ratio R to be set is obtained from the nitrification target nitrogen concentration which is a measured value of the raw water nitrogen concentration and the target treated water nitrogen concentration. Moreover, the target treated water is calculated by calculating the nitrification target nitrogen concentration, which is the nitrogen concentration of the raw water, from the treated water nitrogen concentration, which is a measured value of the nitrogen concentration of the permeated water, and the nitrification liquid circulation ratio R at that time. The nitrification liquid circulation ratio R for obtaining the nitrogen concentration is obtained. Further, if the nitrogen concentration of the water to be treated in any of the denitrification step, the nitrification step, and the membrane separation step and the nitrification solution circulation ratio R at that time are known, the treated water nitrogen concentration or Nitrogen concentration is determined.

  Therefore, as described above, the nitrogen concentration measuring device 33 may be arranged to measure the nitrogen concentration of the organic waste water that is the raw water, and is any one of the denitrification tank 20, the nitrification tank 21, and the membrane separation tank 22. It may be arranged so as to measure the nitrogen concentration of the water to be treated, or it may be arranged so as to measure the nitrogen concentration of the membrane permeated water separated in the membrane separation tank 22. It may be arranged to measure both nitrogen concentrations.

The nitrogen concentration measuring device 33 can use a TN meter that can measure the total nitrogen concentration, an NH ₄ -N meter that can measure the ammonia nitrogen concentration, and a NO ₃ -N meter that can measure nitrate nitrogen. It is preferable to use an NH ₄ -N meter for NO and a NO ₃ -N meter for membrane permeate.

  When the target value of the treated water nitrogen concentration is determined, the target flow rate calculation unit calculates the nitrification liquid circulation ratio R toward the target value, and the return flow rate adjustment unit adjusts the return flow rate via the first return mechanism A. The

  However, for example, if the nitrification liquid circulation ratio R is decreased from winter to spring when the dissolved inorganic nitrogen, which is one of the nutrient salts in seawater, decreases, and the nitrogen concentration of the treated water is increased, the membrane separation tank 22 is treated. The inflow amount of water, that is, the inflow amount of the mixed solution of raw water and activated sludge is reduced, and the gradient of the activated sludge concentration around the submerged membrane separation device 23 arranged along the flow of treated water is increased. There is a risk that the membrane separation performance may be hindered.

  As already explained, if the MLSS concentration in the entire treatment process is lowered so that the MLSS concentration on the downstream side along the flow of the water to be treated in the membrane separation process does not become too high, especially in the membrane separation process where the MLSS concentration is low. On the upstream side, undecomposed ammonia or the like flows out into the permeated water of the membrane, or fouling of the separation membrane due to undegraded poorly soluble components, polymer solutes, etc. occurs and the separation membrane is blocked.

  In the membrane separation tank 22 having a long flow path in the flow direction of the water to be treated and a plurality of submerged membrane separation devices 23 installed in the flow direction of the water to be treated, the difference in MLSS concentration depending on the position in the flow path is more Become prominent.

  In preparation for such a case, the organic wastewater treatment apparatus according to the present invention is configured to return part of the water to be treated in the membrane separation tank 22 to the nitrification tank 21 via the second return mechanism B. ing.

Assuming that the MLSS concentration on the most downstream side of the membrane separation tank 22 is X, the MLSS concentration of the water to be treated flowing into the membrane separation tank 22 is obtained by the following Equation 3. R is the nitrification solution circulation ratio and r is the internal circulation ratio. The flow rate of the membrane permeated water filtered by the submerged membrane separation device 23 and the flow rate of the raw water are assumed to be approximately the same value.
MLSS concentration = (R + r) / (R + r + 1) * X Equation 3

  According to Equation 3, the MLSS concentration flowing into the upstream portion of the membrane separation tank 22 is determined by the nitrification liquid circulation ratio R and the internal circulation ratio r, and the nitrification liquid circulation ratio R is decreased to increase the treated water nitrogen concentration. However, when the internal circulation ratio r is increased, an increase in the difference between the MLSS concentration of the water to be treated flowing into the upstream portion of the membrane separation tank 22 and the MLSS concentration of the water to be treated flowing out of the downstream portion of the membrane separation tank 22 is suppressed. Obviously it can be. The value of R + r is called the total circulation ratio.

  That is, according to the organic wastewater treatment apparatus of the present invention, fouling of the membrane separation apparatus 23 used in the membrane separation tank 22 is avoided even if the nitrification liquid circulation ratio R to the denitrification tank 20 is set to a small value. become able to.

  By adjusting the overall circulation ratio, the nitrogen concentration of the permeated water (processed water nitrogen concentration) and the MLSS concentration distribution in the membrane separation tank 22 will change. The case where the MLSS concentration at the most downstream side of the tank 22 is fixed at 10,000 mg / L will be specifically described as an example.

When R = 3 and r = 0 In this case, the treated water nitrogen concentration is 7.5 mg / L, and the MLSS concentration of the treated water flowing into the upstream portion of the membrane separation tank 22 is 7,500 mg / L. The MLSS concentration distribution ratio in the upstream portion and the downstream portion of the tank 22 is suppressed to 1: 1.3.
When R = 1 and r = 0 In this case, only the nitrification liquid circulation ratio R is decreased in order to increase the nitrogen concentration of the treated water. In this case, the nitrogen concentration of treated water rises to 15 mg / L from Equation 1, but the MLSS concentration of treated water flowing into the upstream portion of the membrane separation tank 22 becomes 5,000 mg / L, and the upstream portion of the membrane separation tank 22 And the MLSS concentration distribution ratio in the downstream portion is 1: 2, and the MLSS concentration gradient of the membrane separation tank 22 along the flow-down direction of the water to be treated becomes too large. Further, there is a possibility that fouling of the separation membrane due to the inflow of undecomposed components in the raw water into the membrane separation tank 22 may occur.
When R = 1 and r = 2 In this case, the nitrification liquid circulation ratio R is decreased and the internal circulation ratio r is increased in order to increase the concentration of treated water nitrogen. In this case, the nitrogen concentration of the treated water rises to 15 mg / L, the MLSS concentration of the water to be treated flowing into the membrane separation tank 22 becomes 7,500 mg / L, and the MLSS concentrations at the upstream and downstream parts of the membrane separation tank 22 The distribution ratio is 1: 1.3, and the MLSS concentration gradient of the membrane separation tank 22 along the flow-down direction of the water to be treated can be suppressed to the same value as <when R = 3, r = 0>.

  As another example, how the MLSS concentration distribution in the membrane separation tank 22 changes in a case where the nitrogen concentration to be nitrified is fixed at 30 mg / L and the MLSS concentration in the nitrification tank 21 is fixed at 8,000 mg / L. This will be explained in detail.

When R = 4, r = 0, and R = 1, r = 3 In these cases, the MLSS concentration of the water to be treated in the upstream portion of the membrane separation tank 22 is 8,000 mg / L, and membrane separation The MLSS concentration of the water to be treated at the most downstream of the tank 22 is 10,000 mg / L, and the MLSS concentration distribution ratio at the upstream portion and the downstream portion of the membrane separation tank 22 is suppressed to 1: 1.25.
When R = 1 and r = 0 In this case, the MLSS concentration of the water to be treated at the most downstream of the membrane separation tank 22 rises to 16,000 mg / L, and the membrane separation stops with the accumulation of sludge on the separation membrane. There is a risk of doing.
The treated water nitrogen concentration is the same as in the previous example.

  As in this embodiment, in the membrane unit 24 employing a separation membrane made of a flat membrane-shaped microfiltration membrane, the MLSS concentration is preferably maintained in the range of 6,000 to 15,000 mmg / L. As described above, even if the nitrification liquid circulation ratio R is reduced, by increasing the internal circulation ratio r, the range of the MLSS concentration distribution in the membrane separation tank 22 can be adjusted to a preferable range while increasing the nitrogen concentration of the treated water. It is.

  That is, the control unit 30 uses the nitrogen concentration acquired by the nitrogen concentration measuring device 33 as an index, and the treated water returned through the first return mechanism A so that the nitrogen concentration of the membrane permeated water becomes the target nitrogen concentration. The flow rate is adjusted, and the flow rate of the water to be treated returned through the second return mechanism B is adjusted so that the flow rate of the water to be treated flowing into the membrane separation tank 22 becomes the target flow rate.

  When the MLSS concentration distribution in the membrane separation tank 22 exceeds 1.5 times between the upstream portion and the downstream portion, fouling due to undecomposed components in the upstream portion, or accumulation of activated sludge in the downstream portion, Membrane separation may not be possible. Therefore, it is necessary to set the ratio of the most upstream MLSS concentration (R + r + 1) / (R + r) derived from Equation 3 to 1.5 or less. That is, R + r + 1 ≧ 3 must be satisfied.

  Further, the increase in the total circulation ratio and the nitrification liquid circulation ratio leads to an increase in energy required for circulation and a denitrification treatment failure due to bringing dissolved oxygen in the treated water into the denitrification tank 20, so The MLSS concentration ratio (R + r + 1) / (R + r) needs to be 1.2 or more. That is, R + r + 1 ≦ 6 must be satisfied.

  From the above, in the present embodiment, it is preferable to set the target value of R + r + 1 within the range of 3-6. That is, the nitrification liquid circulation ratio R is adjusted using the nitrogen concentration as an index, and the internal circulation ratio r is adjusted so that R + r + 1 becomes a target value determined from the above consideration.

  As described above, when the target value of R + r + 1 is set to 3 to 6, the target flow rate of the water to be treated flowing into the membrane separation tank 22 is at least three times the inflow amount of the organic waste water to be treated and This means that the range is 6 times or less.

  At this time, the upstream and downstream MLSS concentration distribution ratio in the membrane separation tank 22 can be within a range from 1: 1.2 to 1: 1.5, and the occurrence of fouling of the separation membrane can be suppressed. . A separation membrane that can cope with the MLSS concentration distribution ratio in the range from 1: 1.2 to 1: 1.5 is standard and easily available at low cost. Furthermore, since the control range of the flow rate of the water to be treated mixed with the activated sludge transferred to the membrane separation tank 22 is relatively wide with a range of 3 to 6 times the flow rate of the organic waste water to be treated, Less difficult to control the flow rate.

  Similarly, the target value of R + r + 1 may be set to a value in the range of 3-6. That is, the target flow rate of the water to be treated flowing into the membrane separation tank 22 may be a constant value that is not less than 3 times and not more than 6 times the inflow amount of the organic waste water to be treated. This is because the MLSS concentration distribution ratio can be set to a value that facilitates drawing out the performance of the separation membrane to be used, so that the occurrence of fouling of the separation membrane can be more effectively suppressed.

  Note that the preferable range of the MLSS concentration ratio (R + r + 1) / (R + r) is not limited to the above numerical range, and can be set as appropriate depending on the characteristics of the separation membrane used in the membrane separation device 23.

Hereinafter, another embodiment will be described.
In the above-described embodiment, an example in which the second return mechanism B is configured to return the water to be treated left without being subjected to the membrane separation treatment in the membrane separation tank 22 to the upstream of the nitrification tank 21 has been described. The second return mechanism B only needs to be configured to return the water to be treated that has not been subjected to the membrane separation treatment in the membrane separation tank 22 to the subsequent stage from the denitrification tank 20, and return it to the downstream of the nitrification tank 21, or You may comprise so that it may return to the upstream of the membrane separation tank 22. FIG.

  In addition, if it comprises so that it may return to the upstream of the membrane separation tank 22, since the MLSS density | concentration of the nitrification tank 21 will become the same as the denitrification tank 20, the required oxygen amount of the nitrification tank 21 will reduce and it will become energy saving.

  By the organic wastewater treatment device 1 described above, the organic wastewater containing nitrogen is mixed with activated sludge in the order of at least the denitrification step, the nitrification step, and the membrane separation step using the submerged membrane separation device. An organic wastewater treatment method for biological treatment as treated water is executed.

  More specifically, a first return process for returning the treated water that has undergone the membrane separation process to the denitrification process, and a second return process for returning the treated water that has undergone the membrane separation process to the process after the denitrification process, Organic waste water, any treated water, and treated water returned in the first returning process using the first returning mechanism A using as an index the nitrogen concentration of any of the membrane permeated water separated in the membrane separating process. And a return flow rate adjustment step of adjusting the flow rate of the water to be treated returned in the second return step using the second return mechanism B is executed.

  The return flow rate adjustment step uses the nitrogen concentration as an index to adjust the flow rate of water to be treated returned in the first return step so that the nitrogen concentration of the membrane permeate becomes the target nitrogen concentration, and flows into the membrane separation step. This is a step of adjusting the flow rate of the water to be treated returned in the second return step so that the flow rate of the treated water becomes the target flow rate.

  In addition, the target flow rate is adjusted to a range that is not less than 3 times and not more than 6 times the inflow amount of the organic wastewater to be treated, or the target flow rate is not less than 3 times the inflow amount of the organic wastewater to be treated and It is adjusted to a constant value in the range of 6 times or less.

Still another embodiment will be described.
Any of the organic wastewater treatment devices described above is further provided with an MLSS concentration measurement device that measures the MLSS concentration of the water to be treated in the membrane separation tank 22, and the control unit 30 returns it via the first return mechanism A. The flow rate of the water to be treated is adjusted by using the nitrogen concentration acquired by the nitrogen concentration measuring device 33 as an index, and the flow rate of the treated water returned via the second return mechanism B is adjusted by the MLSS concentration measuring device. It is preferable that the MLSS concentration is adjusted according to the flow rate of the water to be treated returned via the first return mechanism A so that the acquired MLSS concentration is within a predetermined range. What is necessary is just to set the target value of MLSS density | concentration suitably according to the property of waste_water | drain, and the specification of a separation membrane.

  In the embodiment described above, a plurality of membrane separation devices 23 are immersed in the membrane separation tank 22 along the flow-down direction of the treated water, and the first return mechanism A removes the treated water from the downstream side of the membrane separation tank 22. Although the example provided with the return path which returns to the nitriding tank 20 was demonstrated, it is especially suitable when it is the long membrane separation tank 22 whose flow direction length of to-be-processed water is ten times or more of the horizontal width of the membrane separation tank 22. .

  In a large-scale sewage treatment facility, a long channel through which water to be treated flows is secured in the membrane separation tank 22. For example, an elongate shaped extruded flow reaction tank housing structure is the case. In such a facility, a plurality of submerged membrane separation devices 23 are disposed in the membrane separation tank 22 along the flow direction of the organic waste water to be treated, and the first return mechanism is provided in the membrane separation tank 22. Since the return path for returning the treated water from the downstream side to the denitrification tank 20 is provided, the problem of fouling of the separation membrane is suppressed, and at the same time, further improvement in nitrogen removal efficiency can be expected.

  The present invention is also suitably used for a membrane separation tank 22 in which membrane separation devices 23 are immersed in multiple stages along the direction intersecting with the flow-down direction of the water to be treated, and the MLSS concentration gradient increases in the membrane separation tank 22. be able to.

  In the above-described embodiment, the example in which the first return mechanism A and the second return mechanism B are each provided with the pumps 26 and 27 and the individual return paths 28 and 29 is described. The configuration of the two-return mechanism B is not limited to such a mode.

  For example, as shown in FIG. 2, a single nitrifying liquid circulation pump 26 is provided in the membrane separation tank 22, and a common return path is connected to the individual first return path 28 and second return path 29 via a three-way valve 36. You may comprise so that branch supply may be carried out.

  In the above-described embodiment, an example in which the nitrification tank 21 and the membrane separation tank 22 are configured independently has been described. However, as illustrated in FIG. 3, the nitrification tank 21 and the membrane separation tank 22 are configured integrally and are to be processed. The upstream side of the water may function as the nitrification tank 21, and the downstream side may function as the membrane separation tank 22.

  As shown in FIG. 4, you may comprise so that the anaerobic tank 37 may be further provided in the upstream of the denitrification tank 20, and the phosphorus component contained in raw | natural water may be removed. A denitrification liquid circulation pump 38 is installed in the denitrification tank 20, and one of the water to be treated in the denitrification tank 20 is connected to the denitrification liquid circulation pump 38 and the third return path 39 connected to the denitrification liquid circulation pump 38. What is necessary is just to comprise so that a part may be returned to the anaerobic tank 37. FIG.

  The phosphorus compound contained in the raw water flowing into the anaerobic tank 37 is dissolved and released into the water to be treated as normal phosphoric acid by microorganisms in an oxygen-free state. The released orthophosphoric acid is taken into the aerobic microorganisms in the nitrification tank 21 and the membrane separation tank 22 and is extracted as excess sludge.

  Each of the above-described embodiments is an example of the present invention, and the present invention is not limited by the description. The specific configuration of each part can be appropriately changed and designed within the range where the effects of the present invention are exhibited. Needless to say. Further, any one or a plurality of the above-described embodiments may be appropriately combined.

1: Organic waste water treatment device 20: Denitrification tank 20a: Agitation mechanism 21: Nitrification tank 22: Membrane separation tank 23: Immersion type membrane separation apparatus 24: Membrane unit 25: Aeration apparatus (aeration apparatus)
26: Nitrification liquid circulation pump 27: Internal circulation pump 28: First return path 29: Second return path 30: Control unit 31: Suction pump 31a: Header pipe 32a, 32b: Flow meter 33: Nitrogen concentration measuring device 34: Raw water Inflow path 35: Release path 36: Three-way valve 37: Anaerobic tank 38: Denitrification liquid circulation pump 39: Third return path A: First return mechanism B: Second return mechanism

Claims (10)

Organic wastewater treatment method for biologically treating nitrogen-containing organic wastewater as treated water mixed with activated sludge in the order of at least a denitrification step, a nitrification step, and a membrane separation step using a submerged membrane separation device Because
A first return step for returning the treated water remaining without being taken out as membrane permeate in the membrane separation step to the denitrification step;
A second return step for returning the water to be treated which has not been taken out as membrane permeate in the membrane separation step to a step after the denitrification step;
The flow rate of water to be returned in the first return step, using as an index the nitrogen concentration of any one of the organic waste water, any water to be treated, and the membrane permeated water separated in the membrane separation step, and An organic wastewater treatment method including a return flow rate adjustment step of adjusting a flow rate of water to be treated returned in the second return step.   The return flow rate adjustment step uses the nitrogen concentration as an index to adjust the flow rate of the water to be treated returned in the first return step so that the nitrogen concentration of the membrane permeated water becomes a target nitrogen concentration, and the membrane separation The method for treating organic waste water according to claim 1, wherein the process is a step of adjusting the flow rate of the water to be treated returned in the second return step so that the flow rate of the water to be treated flowing into the step becomes a target flow rate.   The organic wastewater treatment method according to claim 2, wherein the target flow rate is adjusted to a range of 3 to 6 times the inflow of the organic wastewater to be treated.   The organic wastewater treatment method according to claim 2, wherein the target flow rate is adjusted to a constant value in a range of 3 to 6 times the inflow of the organic wastewater to be treated. Organic waste water comprising at least a denitrification tank, a nitrification tank, and a membrane separation tank provided with a submerged membrane separation device for biological treatment of organic waste water containing nitrogen as treated water mixed with activated sludge A processing device comprising:
A first return mechanism for returning the water to be treated in the membrane separation tank to the denitrification tank;
A second return mechanism for returning the treated water in the membrane separation tank to the subsequent stage from the denitrification tank;
A nitrogen concentration measuring device that measures the nitrogen concentration of any one of the organic waste water, any water to be treated, and membrane permeated water taken out by the submerged membrane separator;
Using the nitrogen concentration acquired by the nitrogen concentration measuring device as an index, the flow rate of water to be treated returned via the first return mechanism and the flow rate of water to be treated returned via the second return mechanism are adjusted. A control unit;
Organic wastewater treatment equipment equipped with.   The control unit uses the nitrogen concentration acquired by the nitrogen concentration measuring device as an index, and the flow rate of water to be treated that is returned via the first return mechanism so that the nitrogen concentration of the membrane permeated water becomes the target nitrogen concentration. The organic waste water according to claim 5, wherein the flow rate of the treated water returned through the second return mechanism is adjusted so that the flow rate of the treated water flowing into the membrane separation tank becomes a target flow rate. Processing equipment.   The organic wastewater treatment apparatus according to claim 6, wherein the target flow rate is adjusted to a range of 3 to 6 times the inflow of the organic wastewater to be treated.   The organic wastewater treatment apparatus according to claim 6, wherein the target flow rate is adjusted to a constant value in a range of 3 to 6 times the inflow of the organic wastewater to be treated. An MLSS concentration measuring device for measuring the MLSS concentration of the water to be treated in the membrane separation tank;
The control unit uses the nitrogen concentration acquired by the nitrogen concentration measuring device as an index, and the flow rate of water to be treated that is returned via the first return mechanism so that the nitrogen concentration of the membrane permeated water becomes the target nitrogen concentration. And the second return mechanism according to the flow rate of the water to be treated returned through the first return mechanism so that the MLSS concentration acquired by the MLSS concentration measuring device falls within a predetermined range. The organic waste water treatment apparatus of Claim 5 which adjusts the flow volume of the to-be-processed water returned via. In the membrane separation tank, a plurality of the immersion type membrane separation devices are arranged along the flow-down direction of the water to be treated,
The organic wastewater treatment apparatus according to any one of claims 5 to 9, wherein the first return mechanism includes a return path for returning the water to be treated downstream of the membrane separation tank to the denitrification tank.

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